The present invention relates to a decision feedback equalizer for equalizing waveform distortion of a received signal affected by fading in radio wave transmission medium.
In case of occurrence of fading in radio wave transmission medium, there may appear increase of intersymbol interference effected by transmission distortion and increase of noise effected by drop of receiving electric field intensity. The equalizer is useful for suppressing intersymbol interference, but not useful for suppressing interference, such as noise generated with no relation to any transmitted radio wave. Especially, in a decision feedback equalizer, it may be occurred that interference by noise increases affect to a main signal due to transfer of error data. This transfer of error will be described hereinafter.
Although the decision feedback equalizer can suppress intersymbol interference by feeding back decided data, it has undesirable tendency such that errors increase by feeding back decision data in a case where the decision error rate is not negligible. It is now assumed that a symbol of a received signal is affected by waveform distortion and decision error appears in excess of the allowance of equalization function. In this case, decision error data are fed back through a feedback tap, and waveform equalization operation is carried out on the base of its decision error data for a next symbol. When waveform equalization operation is carried out on the base of the decision error data, decision error appears for waveform equalization operation of the next symbol in spite of non existence of noise. Since decision error is also fed back for each of further succeeding symbols, chain error reaction will be liable to appear.
As mentioned above, the decision feedback equalizer has a defect such that, since a decision error of a symbol caused by noise generated with no relation to any transmitted radio wave affects to a normal symbol not affected by noise interference because of the above mentioned feed back operation of decision data, transfer of error for causing the above mentioned chain error reaction will be liable to appear.